Infrared headlights such as these are used, for example, as illuminators for night-vision systems, which will also be intended in the future for use in motor vehicles.
EP 1 072 841 A2 discloses an infrared headlight in which an incandescent lamp is used as the infrared radiation source, whose incandescent filament emits both infrared radiation and light in the visible range during operation. In the known solution, the infrared headlight has a parabolic reflector, which deflects the infrared radiation to the desired direction and transmits the visible radiation. The reflector opening is covered by a filter disk, and the lamp vessel (which surrounds the incandescent filament) of the infrared emitter is provided with a light-reflective coating in a dome area. The filter disk, which is opaque to light in the visible range, represents a planar filter which may be designed either on the basis of the absorption principle or on the basis of the interference principle. A solution such as this has the following disadvantages: infrared planar filters based on the absorption principle are not suitable for temperatures above 250° C., and in some cases also contain cadmium. For these reasons, planar filters such as these are not suitable for automotive applications. Infrared planar filters based on the interference principle are suitable only for a very limited aperture of the headlight because the filter edge is shifted towards shorter wavelengths by the incidence angles (which increase towards the edge) of the radiation emitted from the infrared emitter and this can result, for example, in disturbing red light. In practice this means that it is not possible to cover the entire outlet area of a conventional headlight with the filter, in order to avoid residual red light, and, instead, an annular area must be left free on the outside, through which white light is emitted.
DE 39 32 216 A1 discloses an illumination device for automotive applications, which can be used both as an infrared headlight and as a main beam. The illumination device has a reflector in which a light source is inserted. Furthermore, a filter is also inserted in the reflector, through which infrared radiation can pass and which reflects radiation in the visible range towards the light source. In the main beam mode, the filter is moved with respect to the light source such that it is ineffective, so that all of the radiation is reflected via the reflector towards the reflector opening. When dipped lights are selected, the filter is moved over the light source such that the illumination device emits only infrared radiation.
The disadvantage of a solution such as this is that the cylindrical filter which is moved over the illumination device can result in thermal problems, so that the lamp is either too small for the main beam mode, or becomes too hot in the infrared mode. Furthermore, it has been found that, even with this illumination device, disturbing red light is created as a result of high incidence angles on the cylindrical interference filter. A complex mechanism is required to move the filter. The installation depth of the headlight is considerably increased, because the entire filter must be located in front of the lamp in the main beam mode. A slot is required in the reflector, which reduces the effective reflector area and thus the efficiency of the arrangement.